If you are using Xfce then Thunar is your default file manager and you can set it to open files with one click here.....Open Thunar then click the "Edit" menu then "Preferences" then the "Behaviour" tab .

If you are using LXDE like I think I saw in one of your threads then pcmanfm is your default file manager.It is almost exactly the same...Open Pcmanfm then "Edit" --> "Preferences" --> Here under the heading of "Behaviour" you can select to open files with a single click.